[Elecraft] Antenna Question

[Ron D' Eau Claire]

>...I'm thinking of a 67ft wire from the roof
> to a tree,
> end feed with about 30ft ladder line comming into the upper floor "shack".
> The ARL handbook indicates a transmatch is needed. ...Any thoughts or
> recommendations will be appreciated.
>> john
> va3jsl@rac.ca

That's sort of a Zepp antenna. They are VERY frequency-sensitive if you want
to avoid a lot of radiation from the feeders.

The original Zepp used a 1/2 wave radiator and a 1/4 wave long feeder. You
provide balanced feed to the open wire line at the rig. At the other end ONE
side of the feeder goes to the radiator and the other side is insulated and
connected to nothing at all.

The 1/2 wave radiator shows a high impedance at the end where it is fed, so
very little current is drawn from the feeder. It is a classic "voltage" feed
with very high r-f voltages at that point. Since the current in the side of
the feeder going to the antenna is very low, and the current on the side of
the feeder that connects to nothing is also very low (only whatever leakage
there is through the insulator), the currents in the feeders will be fairly
well balanced, preventing radiation from the feeder. The 1/4 wave long
feeder transforms that very high impedance into a low impedance at the ATU
in the shack, which keeps the r-f voltages there low.

The moment you move from the frequency where the radiator is exactly 1/2
wave long it will start to 'draw current' from the feeder. Since only one
side of the feeder is connected to anything, that will unbalance the
currents in the feedline and it will begin to radiate.

Still, it can be an effective antenna, although at any but the design
frequency it is more like a simple 'end fed' wire with the antenna coming
right down into the shack.

Also, on some frequencies you will develop very high r-f voltages at the ATU
end of the feeder. One time I literally burned up a 300 watt balun (and
melted a nylon binding post) running only 10 watts! I was using an end fed
Zepp on various bands and hit a combination that put the voltage loop (high
impedance point) at the ATU. Even at 10 watts there was plenty of r-f
voltage to arc between windings on the balun and melt the insulation on the
binding post.

You might do as well or better by simply end-feeding the wire since you are
already on an upper floor. The KAT2 should handle that just fine. If it has
trouble on one band, just change the antenna length a couple of feet and you
should be able to find a length that it can handle on all bands. You will
want to use a tuned counterpoise - a 1/4 wave long wire for each band - one
end connected to the K2 case and the other end insulated. They can be run
anywhere as long as they are insulated. On bands where you have a high
impedance at the rig they will keep the K2 at r-f ground and avoid problems
with 'r-f in the shack'. On bands where the antenna presents a low impedance
at the rig, the counterpoise is essential to provide an efficient r-f
ground. Without it, most of your power will be lost in the poor 'ground'
connection instead of being radiated.

Be careful though, the far end of the counterpoise can cause r-f burns if
someone touches it.

An alternative to separate counterpoise wires is to use an "Artificial
Ground" like MFJ sells. It's a simple L-match and r-f ammeter that you use
to tune a single counterpoise wire to 1/4 wavelength at the operating
frequency. That provides a good 'r-f ground' at the rig end. I have used one
with very good results. I have used one successfully with a counterpoise
wire that was as little as 1/16 of a wavelength long on the operating
frequency (16 feet on 80 meters) although I believe that Moxon suggests
about 1/8 wave as a suitable 'minimum' counterpoise length.

If you are where must have a fairly long run before your antenna is out of
the shack and 'in the clear', you might want to consider a balanced antenna
instead - a doublet fed with the open wire line in the center. With balance
feed you will avoid radiation on the feeders as long as the two halves of
the antenna are the same length and fed from a well-balanced source. It will
work well on any frequency down to where the 'flat top' is 1/8 wavelength
long.

One of my favorite books for all manner of interesting and effective wire
antennas and tuner designs is a little book by John D. Heys, G3BDQ, called
"Practical Wire Antennas - Effective HF Designs for the Radio Amateur".
Published by the Radio Society of Great Britain (ISBN 0 900612 87 8) it is
available from several sources, including the ARRL here in the USA.

Ron AC7AC
K2 # 1289